I first saw it as a kid, never forgot it, and recently bought it on DVD. Yeah, it's a silly premise, but that's what was magical about "Twilight Zone" when it worked. For instance, the episode with Anne Francis ("After Hours"), or the idea for the episode, is ridiculous when viewed in the cold light of day, but that episode alternately haunted and terrified me for years, and when I watch it now, I can well understand why. The genius of Rod Serling was to take the mundane and make it mysterious, and...in some cases...miraculous.
"Five Characters In Search Of An Exit" isn't quite as powerful as "After Hours" but it's pretty potent stuff, thanks mainly to the actors involved, particularly the late, great William Windom. Windom was an always reliable character actor who could make good material great and bad material better. Also exceptionally good in this episode is Australian actor Murray Matheson, who plays the sardonic, defeated clown. Susan Harrison as the ballet dancer was very pretty, and the scene at the end, where a single tear emerges from "her" eye (or the eye of...but I won't spoil it for those who haven't seen it) was simultaneously haunting, chilling, and sad, despite the "upbeat" voice over.
If the episode has a fault, it's the occasionally overblown verbiage Serling puts into the mouths of his characters. Serling was definitely a genius, and when he was on the top of his game, his ideas and his scripts were unforgettable. But occasionally, when he wasn't able to reign himself in, the dialogue could be cringe-inducing; at his worst, Serling's characters don't talk to each other--- they pontificate at each other in long, windy, self-righteous monologues. Thankfully, that only happens once or twice in this episode (and even when it does, it doesn't rankle quite as much thanks to the skill of the actors involved), and what's good about it certainly outweighs any negativity.
Not necessarily one of the "best" episodes, but it made an indelible impression on me, and it's certainly one that the viewer won't forget anytime soon.